The present invention relates to a method of filling voids in a substrate with a hot melt adhesive composition. More particularly, it refers to a method of filling voids with a composition comprising a block copolymer containing glass fibers and a particulate numeral reinforcing agent.
Hot melt adhesives are well known in the prior art. These materials are conveniently applied to a substrate in the molten state and upon cooling form an adhesive bond. However, a deficiency common to most of the hot melt adhesives of the prior art is their tendency to soften and flow at elevated temperatures, as, for example, 70.degree. to 100.degree. C. with a resulting loss of bond strength. Consequently, these materials are not suitable for use over a broad temperature range.
Attempts to upgrade the softening and flow temperatures have involved using very high molecular weight resinous materials and/or crosslinking of the resin. These methods have resulted in materials with higher softening points and flow temperatures. However, in most cases the resulting material was not adapted to thermal processing because its higher molecular weight and/or crosslinked structure engendered extremely high application viscosity. Thus, these materials were not suitable for use as hot melt adhesives.
In the manufacture and repair of metal bodies such as automobiles and appliances, solder compositions containing lead are frequently used to fill cavities and voids. These lead solders are extremely dense and can add a significant increment to the weight of the metal body. They present a health hazard which mandates special handling to protect workers engaged in the soldering and cavity filling operations. Curable adhesives such as epoxies are generally unsatisfactory for such cavity and void filling applications because they require careful metering of the components to provide good physical properties and bond strength, because they take too long to cure to a sandable state and because they have rather poor weather resistance. Conventional hot melt adhesives are also unsatisfactory for cavity and void filling applications because they cannot be sanded rapidly at assembly line speed, they do nto readily accept paint, exhibiting "bleed-through", and they do not withstand the high temperatures necessary for the subsequent cure of paint overcoats. "Bleed-through" or "telegraphing" is the term used to describe the revelation of difference in composition of the substrate when it has been painted, caused by a difference in reflectivity between the painted metal and the painted adhesive composition.
U.S. Pat. No. 3,650,999 discloses block copolymer comprising hard polyester segments and soft polyamide segments having improved adhesion and high temperature performance obtained by reacting a crystalline polyester, a C.sub.18 to C.sub.54 polycarboxylic acid and a primary diamine. This poly(ester-amide) in common with other hot melt adhesives has deficiencies in creep resistance at temperatures above 150.degree. C. in the range up to 205.degree. C. and above and in shrinkage when the hot melt is cooled to room temperature after application. These deficiencies have been covercome to a considerable degree by incorporating a metal powder into the block copolymer to yield a cavity filling composition which possesses good sandability and paint acceptance. However, the metal powder copolymer composition can lack adequate impact resistance especially at low temperatures and can sag excessively at elevated temperatures. Attempts to improve the impact resistance by introducing an energy-absorbing rubber reinforcement were generally unsuccessful and added a further complication of blinding of the sanding disc, making sanding extremely difficult.
The present invention is a method of filling a cavity in a substrate which comprises applying an adhesive composition as a hot melt to fill the cavity, cooling the adhesive composition below the crystallization temperature of the block copolymer and sanding the adhesive composition to provide a surface even with the surrounding substrate.
The adhesive composition comprises a block copolymer, a particulate mineral reinforcing agent and glass fiber; wherein the block copolymer is selected from the group consisting of copolyesters, copolyamides, copoly(esteramides) and copoly(ether-esters) melting at a temperature of at least about 150.degree. C., having from about 30 to about 70 weight percent of hard segments and from about 70 to about 30 weight percent of soft segments, wherein the weight ratio of block copolymer to aluminum powder and glass fiber is in the range of about 3:7 to about 3:2, wherein the weight ratio of block copolymer to glass fiber is at least about 3:4 and wherein the weight ratio of particulate mineral reinforcing agent to glass fiber is in the range of about 1:3 to about 9:1.
The adhesive composition has improved impact resistance at low temperatures, is less dense and toxic than lead solder, forms a strong bond to metal and painted metal substrates, withstands extremes of humidity and temperature, has sag resistance at elevated temperatures, is readily trowelled to fill a cavity, sets rapidly to a sandable state, is easily sanded smooth and accepts paint without "bleed-through".
The block copolymer of the adhesive compositions of the present invention is selected from the group consisting of copolyesters, copolyamides, copoly(ester-amides) and copoly(ether-esters) melting at a temperature of at least about 150.degree. C., having hard segments and soft segments to provide a balance of physical properties and processability. These segments are considered to exist in microscopic domains within the bulk mass of copolymer resin to provide a heterephase system in which the copolymer will have physical properties reflecting the properties which the respective segments would manifest independently. By control of the relative size, proportions, crystallinity and crystal melting points of the segments, the tack, open time and bond strength of the adhesive can be controlled. The hard segments contribute crystalline blocks to the copolymer so that optimum bulk physical properties such as tensile strength and stiffness can be achieved without incurring the disadvantage of high processing viscosity.